This work is a study of several events that occur preparatory to and at the time of fertilization. The insertion of cortical granule membrane protein into the surface of the egg to make a mosaic plasma membrane will be investigated by surface labeling techniques. The peroxidative system activated at the time of fertilization that is dependent upon release of an ovoperoxidase from cortical granules will be studied, with special attention to the mechanism of synthesis of hydrogen peroxide, the mechanism of action of the ovoperoxidase, and the role of the enzyme in the alteration of the glycocalyx and in prevention of polyspermy. The acrosome reaction of sperm will be studied in an attempt to define a linear sequence of events responsible for triggering this complex reaction. Attention will be directed toward ion movements that accompany the reaciion and their roles in different phases of the acrosome reaction.